U.S. patent number 7,778,709 [Application Number 11/104,389] was granted by the patent office on 2010-08-17 for method and device for using impedance measurements based on electrical energy of the heart.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Eckhard Alt, Maik Gollasch.
United States Patent |
7,778,709 |
Gollasch , et al. |
August 17, 2010 |
Method and device for using impedance measurements based on
electrical energy of the heart
Abstract
A method and a device are disclosed for evaluating the
cardio-circulatory and pulmonary condition of a patient, including
determining the patient's thoracic impedance based on information
solely derived from the electrical energy generated by the
patient's own heart.
Inventors: |
Gollasch; Maik (Berlin,
DE), Alt; Eckhard (Ottobrunn, DE) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
46304315 |
Appl.
No.: |
11/104,389 |
Filed: |
April 11, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050288726 A1 |
Dec 29, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10622184 |
Jul 16, 2003 |
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10155771 |
May 25, 2002 |
6829503 |
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Foreign Application Priority Data
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Oct 1, 2001 [DE] |
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101 48 440 |
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Current U.S.
Class: |
607/35;
607/9 |
Current CPC
Class: |
A61B
5/316 (20210101); A61B 5/0031 (20130101); A61B
5/0537 (20130101); A61N 1/3627 (20130101); A61N
1/36521 (20130101); A61B 2562/0219 (20130101); A61B
5/0809 (20130101) |
Current International
Class: |
A61N
1/378 (20060101) |
Field of
Search: |
;607/17,35,9,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2004/032788 |
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Apr 2004 |
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WO |
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WO 2007/109272 |
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Sep 2007 |
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WO |
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Primary Examiner: Layno; Carl H
Assistant Examiner: Gedeon; Brian T
Attorney, Agent or Firm: IPLM Group, P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of Ser. No. 10/622,184
filed Jul. 16, 2003, now abandoned which is a continuation-in-part
of Ser. No. 10/155,771 filed May 25, 2002 now U.S. Pat. No.
6,829,503 that claims priority of German patent 10148440-2 filed
Oct. 1, 2001 of the same applicant, each of which applications is
incorporated by reference in its entirety herein. Applicant claims
priority of the aforesaid applications with respect to common
subject matter.
Claims
What is claimed is:
1. A method of evaluating a condition of a patient having a heart
and a thoracic impedance, comprising the steps of: detecting an
electrical signal from said heart of said patient using signal
circuitry via electrode means arranged and adapted for sensing said
electrical signal, while using electrical energy generated by said
heart of said patient and no other electrical power for said signal
circuitry; and determining said thoracic impedance based on said
electrical signal.
2. The method as in claim 1 further comprising the step of
optimizing a function of a rate adaptive pacemaker based, at least
in part, of said thoracic impedance.
3. The method as in claim 1 further comprising the step of
optimizing monitoring of a congestive heart failure of said patient
based, at least in part, of said thoracic impedance.
4. The method as in claim 1 wherein said electrical signal is at
least partially representative of an EKG of said patient.
5. The method as in claim 4 wherein said EKG is sensed on an
intracardiac basis.
6. The method as in claim 5 wherein said detecting step is
accomplished, at least in part, by a bipolar electrode.
7. The method as in claim 5 wherein said detecting step is
accomplished, at least in part, by a unipolar electrode.
8. The method as in claim 7 wherein said detecting step is
accomplished, at least in part, with a device adapted to be
implanted in said patient, wherein said detecting step utilizes one
electrode positioned in said heart of said patient and electrically
connected to circuitry within an implanted device and another
electrode constituting an electrically conductive portion of a case
of said device.
9. The method as in claim 4 wherein said detecting step utilizes a
device adapted to be implanted in said patient and utilizes surface
mounted electrodes associated with said device.
10. The method as in claim 4 wherein said detecting step senses
from said electrical signal two EKG derived information signals
reflecting relatively high and low input impedance loads on said
electrode means.
11. The method as in claim 10 wherein said determining step
processes said two EKG derived information signals to obtain
information indicative of short-term and long-term trends of said
thoracic impedance of said patient.
12. The method as in claim 1 wherein said thoracic impedance
comprises impedance from myocardial tissue, fibrous connective
tissue in the heart, pericardium, blood within the heart, fluids
within the intracellular spaces, and the surrounding environment of
connective tissue atria, pulmonary structures, venous structures,
and lung tissue.
13. A method of adjusting a heart rate of a patient having a heart
and thoracic impedance by means of an implantable rate adaptive
pacemaker having a pacing rate, comprising the steps of:
determining said thoracic impedance from said heart of said patient
using signal circuitry while using electrical energy generated by
said heart of said patient and no other electrical power for said
signal circuitry; adjusting said pacing rate of said implantable
rate adaptive pacemaker in response to said thoracic impedance.
14. The method as in claim 13 further comprising the step
optimizing said pacing rate through closed loop optimization.
15. The method as in claim 13 further comprising the step of
adapting a heart rate of a rate adaptive pacemaker based, at least
in part, on said thoracic impedance.
16. The method as in claim 13 further comprising the step of
instantaneously influencing adaptation of said pacing rate on an
ongoing basis within minutes based, at least in part, on said
intrinsic impedance.
17. The method as in claim 13 further comprising the step of
determining said adaptation of said pacing rate on a long term
daily and/or monthly basis based, at least in part, on long term
rate and cardio-circulatory response from said intrinsic impedance
information of said patient.
18. A method of monitoring a cardio-circulatory status of a patient
having a heart and a thoracic impedance with an implantable device,
comprising the steps of: detecting an electrical signal from said
heart of said patient using signal circuitry while using electrical
energy generated by said heart of said patient and no other
electrical power for said signal circuitry; and calculating said
impedance based on said electrical signal.
19. The method as in claim 18 wherein said detecting step is
accomplished subcutaneously.
20. The method as in claim 18 wherein said electrical signal is at
least partially representative of an EKG of said patient and
wherein said calculating step is at least partially accomplished
through differential signal processing of said electrical
signal.
21. The method as in claim 20 further comprising the step of
applying information concerning changes in said impedance to
determine said cardio-circulatory status of said patient.
22. The method as in claim 18, wherein the patient is suffering
from heart failure, further comprising the step of monitoring said
heart failure by performing said calculating step in calculating
said impedance.
23. A method of enhancing function of a body-implantable
defibrillator having sensing electrodes adapted to function in
conjunction with a heart of a patient, comprising the steps of:
detecting an electrical signal generated by said heart of said
patient using signal circuitry while using electrical energy
generated by said heart of said patient and no other electrical
power for said signal circuitry; and determining an impedance
between said sensing electrodes of said defibrillator based, at
least in part, on said electrical signal; and determining changes
in said impedance between said sensing electrodes of said
defibrillator based, at least in part, on said electrical
signal.
24. A device for monitoring a cardio-circulatory status of a
patient having a heart and a thoracic impedance with an implantable
device comprising: signal circuitry means for detecting an
electrical signal generated by said heart of said patient while
using electrical energy generated by said heart of said patient and
no other electrical power for said signal circuitry means; and
means for calculating said thoracic impedance based on said
electrical signal.
25. A method of obtaining information about a cardiac function of a
patient having a heart, comprising the steps of: detecting an
electrical signal generated by said heart of said patient related
to an EKG using signal circuitry while using electrical energy
generated by said heart of said patient and no other electrical
power for said signal circuitry; and continuously processing said
electrical signal during depolarization and repolarization of said
heart of said patient, said electrical signal representing systole
and diastole as different phases of said heart.
26. The method as in claim 25 further comprising the steps of:
analyzing an impedance of said heart and changes of said impedance
with systole from a point close to a T-Wave of said electrical
signal; and deriving information on a diastolic status of said
heart derived from said electrical signal close to an R-Wave of
said electrical signal.
27. The method as in claim 26 further comprising the step of
comparing said impedance at systole with said impedance at diastole
to assess a cardio-circulatory status of said patient.
28. An implantable medical device for monitoring a cardio-pulmonary
status of a patient having a heart and a thoracic impedance,
comprising: signal circuitry means for detecting an electrical
signal generated by said heart of said patient related to an EKG
while using electrical energy generated by said heart of said
patient and no other electrical power for said signal circuitry
means; and means for determining said thoracic impedance based on
said electrical signal related to said EKG.
29. The implantable medical device of claim 28, wherein said
implantable medical device is adapted for subcutaneous
implantation.
30. An implantable device as in claim 28 for monitoring a
cardio-pulmonary status of a patient having a heart, comprising
means for determining a thoracic impedance of said patient from an
EKG signal information acquired using electrical energy generated
by said heart of said patient; and surface mounted electrodes,
operatively coupled to said means for determining, for sensing said
EKG signal information; wherein said device is adapted for
subcutaneous implantation.
31. The implantable medical device as in claim 28 further
comprising signal processing means, operatively coupled to said
means for determining, for deriving information concerning changes
in said thoracic impedance to determine the cardio-pulmonary status
of the patient.
32. The device of claim 28, wherein said implantable medical device
is adapted for cardiac pacing, and further comprising means for
negative feedback closed loop control of said cardiac pacing for
optimization of a pacing rate to match a fitness level of said
patient engaged in physical activity.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to obtaining a measure of a
patient's thoracic impedance, and more particularly to doing so
using information derived from an EKG signal with a device that
relies on electrical energy provided by the patient's own
heart.
Specific resistance of biological materials and impedance
measurements have played a major role in modern medicine. The
electrical conductivity and capacity of disperse systems have been
described as early as 1931 (Fricke et al., The Electric
Conductivity of and Capacity of Dispersed Systems; Physics 1931;
1:106-115). Later, especially in the 1950s and 1960s, significant
interest was directed towards the resistance of biological
materials (e.g., Geddes L. et al., The Specific Resistance of
Biological Material: A Compendium of Data for the Biomedical
Engineer and Physiologist, Medical and Biological Engineering 1967,
5:271-293). The application of impedance and resistance
measurements for cardio-circulatory function by measuring the blood
and body temperature has been studied extensively by Geddes et al.,
Medical and Biological Engineering 1967, 11:336-339). Also,
internal and external whole body impedance measurements have been
used for noninvasive monitoring and determination of cardiac output
(Carter et al., Chest 2004, 125:1431-1440). In addition, the
feasibility of using intracardiac impedance measurements has been
evaluated by E. Alt et al. for capture detection in connection with
cardiac pacing (Pace 1992, 15:1873-1879).
Background patents that describe the use of impedance in
conjunction with implantable devices are referenced in U.S. Pat.
No. 5,003,976 to Alt, which describes the cardiac and pulmonary
physiological analysis via intracardiac measurements with a single
sensor. The '976 patent discloses that a single functional
parameter, namely intracardiac impedance, varies both with the
intrathoracic pressure fluctuations following respirations and with
cardiac contraction. This value is representative of both pulmonary
activity and cardiac activity. The finding indicates that this
information derived from intracardiac impedance can be used not
only to monitor the patient's cardiac and pulmonary activity,
condition and cardio-circulatory status, but also, to control the
variability of the rate of an implantable cardiac pacemaker.
U.S. Pat. No. 4,884,576 to Alt et al. discloses a self-adjusting
rate responsive cardiac pacemaker and method based on the
intracardiac signal derived from impedance measurements using an
electrode implanted into the heart. And U.S. Pat. No. 4,919,136,
also to Alt, describes a ventilation controlled pacemaker which
uses the ventilation signal derived from those impedance
measurements with an electrode in the heart to adjust the pacing
rate.
Recently, considerable interest has been focused on the monitoring
of congestive heart failure by means of impedance. U.S. Pat. No.
6,473,640 to Erlebacher describes a system that detects changes in
resistance to a flow of current in the systemic venous system, and
detects changes in impedance to a flow of current through lungs.
The specific signal processing enables a determination of
congestion in the venous or in the pulmonary system by application
of differential signal processing of impedance. Other methods, such
as are described by Combs in U.S. Pat. No. 5,957,861 and Riff in
U.S. Pat. No. 5,876,353, respectively pertain to impedance
monitoring for discerning edema through evaluation of respiratory
rate, and use of implantable medical devices for measuring time
varying physiological conditions, especially edema, and for
responding thereto.
U.S. Pat. No. 6,104,949 to Pitts-Crick relates to a device and a
method used for the diagnosis and treatment of congestive heart
failure. Godie, in U.S. Pat. No. 6,351,667, describes an apparatus
for detecting pericardial effusion, in which a wire probe anchored
to the right heart ventricle and two other wire probes are used to
measure the impedance between the different probes in order to
assess the degree of pericardial effusion.
U.S. Pat. No. 4,899,758 to Finklestein et al describes a method and
apparatus for monitoring and diagnosing hypertension and congestive
heart failure. U.S. Pat. No. 6,336,903 to Brody relates to an
automatic system and method for diagnosing and monitoring
congestive heart failure and the outcomes thereof. U.S. patent
publication 2002-0115939 to Moligan et al describes an implantable
medical device for monitoring congestive heart failure in which
incremental changes in parameter data over time provide insight to
the patient's heart failure state.
The measurement of heart failure becomes of greater clinical
interest and importance as more than 5 million patients in the U.S.
are affected. With deterioration of myocardial function, patients
often require repeated hospitalization. Current methods of
monitoring congestive heart failure cannot reliably predict an
early occurrence of this congestive heart failure; but an
understanding of its occurrence may provide an early indicator of
this adverse event for the patient.
A considerable number of new treatment forms have been introduced
into clinical practice. It had been shown that congestive heart
failure can be treated, not only by drugs, especially Beta
blockers, but also by biventricular pacing. This method makes use
of the exact timing of a stimulus, not only to the right ventricle
or to the septum, but also to the left side of the heart by means
of an electrode which is implanted into the coronary venous
circulation. By these means, the left ventricle can be stimulated
at a time that provides an optimal synchronization of the heart and
improves the mechanical effectiveness of the systole by
synchronizing the depolarization of the right heart, the septum and
the left heart. This avoids the ineffective late contraction of the
left ventricle at a time when the septum depolarization has already
occurred, and the squeezing of the blood by synchronous action of
the septum and left ventricle is no longer present. In addition,
the reduction in mitral valve regurgitation by this type of
resynchronization has been shown.
Studies published at the 2005 meeting of the American College of
Cardiology in Orlando, Fla., USA (CARE-HF study) illustrate that
not only the quality of life of those patients with New York Heart
Association, Heart Failure Class 3 and 4 can be improved, but also
the life expectancy. This recent data show very impressively that
over a 3-year period such biventricular stimulation and the
mortality can be reduced by half in a highly significant manner.
All these new devices improve the survival and quality of life of
patients and have a beneficial effect on re-hospitalization.
Nevertheless, the occurrence of heart failure is still a major
problem for these patients, and it is beneficial to detect such a
heart failure as early as practicable.
A second parameter which plays a major role in patients with
implantable devices such as pacemakers and defibrillators is the
correct adjustment of heart rate. Rate adaptive pacemakers in the
past have provided an open type of correlation between a signal
parameter to adjust the heart rate and the affected heart rate.
However, even multiple sensor parameters that have been used for
adjustment of the pacing rate have not brought the real need of a
patient to clinical practice, mainly a closed-loop monitoring of
heart rate.
In the healthy person, the heart rate is regulated by a very
sophisticated closed loop and negative feedback. Heart rate only
increases to a level with exercise which is physiologically
beneficial. This means that if a patient exercises only mildly, his
heart rate increases proportional to the increase in oxygen uptake
for this person which is a fraction of his maximum exercise
capacity, maximum oxygen uptake and aerobic and anaerobic capacity.
Thus, if someone is well-trained, an external load of 50 watts
might represent only 25% of his/her maximum exercise capacity if
the patient is capable of exercising up to a level of 200 watts.
With this external load of 50 watt the heart rate will increase by
only the fraction that is represented by the patient's resting
heart rate and maximum exercise heart rate. In other words, such a
well-trained person will increase his/her heart rate only by 30-35
beats per minute (bpm). A less capable patient who has a maximum
exercise capacity of 100 watts, will increase his/her heart rate
with the same external load to a higher degree. In that case, the
slope of increase in heart rate depends not only on a fixed
relation of a sensor parameter, such as ventilation or physical
activity or any other physiologic parameter having a suitable
correlation with heart rate, but also on his/her underlying
cardio-pulmonary exercise capacity and condition.
It is therefore a principal aim of the present invention to provide
a novel method to detect a parameter that can control not only the
heart rate in a physiologic appropriate manner, and provide a
closed-loop feedback control for the optimal heart rate of an
exercising patient, but also to monitor the individual status of
the patient under a wide range of physiologic conditions including
normal resting status, congestive heart failure and exercise
states.
Many attempts have been made in the past to use impedance
measurements to derive appropriate signals; however, the past
approaches have involved use of external power sources to power the
device(s) that would monitor and detect impedance. This external
energy can be applied either outside the thorax from a supply
external to the body, or by an implantable device that uses energy
from an electrical battery housed within the device itself.
SUMMARY OF THE INVENTION
It is the aim of the present invention to provide means and methods
to monitor impedance of a patient by using the patient's own heart
as the power source.
It is a further aim of the invention to provide a method of
detecting the thoracic impedance of a person without need for a
battery or other external power source or need for the respective
circuitry to provide the current or voltage for the impedance
measurement that has heretofore limited the availability of energy
to power implantable devices, and accordingly required periodic and
even relatively frequent replacement of the implanted device.
It has been long known that the EKG, which can be derived form the
surface of the patient, represents a voltage generated by the
heart. This voltage is derived from the skin of a patient by means
of electrodes which are attached. The resulting voltage in an EKG
can be detected from different leads. There are bipolar electrodes
which derive a voltage, between for example Lead I the right arm
and the left arm, in the way that the resulting voltage change
between these two electrodes represents the main vector of the
heart in projection to those leads. Therefore, the amplitude is a
measurement of the voltage generated by the heart and the vector.
The input impedance of an external EKG machine is standard in a
range between 1-10 megohms. This means that the input impedance and
resistance of such an amplifier is very high and therefore no
current is shunting through the machine and the voltage always
represents the maximum voltage generated from the energy source,
mainly the heart. Differences in voltages with the current EKG
measurement result from different vectors that project two
different leads on the surface of a patient.
The same holds true for voltages detected with implantable devices
from leads which are situated within the heart or within the thorax
or even implantable devices which have EKG electrodes outside the
thorax, such as EKG loop recorders or devices which are suitable
for monitoring the EKG and congestive heart failure from electrodes
that are situated outside the thoracic cage such as described by
Alt et al in the aforementioned related '771 patent
application.
The underlying principle of the invention may be summarized, for
exemplary purposes, from experiments conducted by the applicants.
The measurements that resulted from placement of standard EKG I,
II, and III leads on the patient were recorded in the presence and
absence of an external load. The amplitude of the EKG signal that
corresponds to the measured voltage is a function of the impedance
of the EKG amplifier.
The underlying theory of the invention is that the heart acts as a
battery. A battery fails when its internal resistance has increased
to a level at which the battery can no longer supply a useful
amount of power to an external load. That same principle applies to
the measurement of electrical energy generated by the heart. That
is, if several loads are applied to the measurement device, which
may be an implantable cardiac pacemaker, a defibrillator, a device
for monitoring the occurrence of heart failure, or a diagnostic
device for monitoring the physical condition of a patient, the same
phenomena can be used to calculate the internal impedance at the
site of measurement. Preferably, the calculation or determination
is of the thoracic (and preferably, intrathoracic) impedance or of
local impedance and/or its relative changes with time for a given
patient.
According to an important aspect of the invention, a method of
evaluating the cardio-circulatory condition of a patient would
include determining the patient's thoracic impedance based on
information solely derived from the electrical energy generated by
the patient's own heart. The intrathoracic impedance information
may be used, for example, to optimize the rate response of a rate
adaptive pacemaker, or to optimize monitoring of the patient's
congestive heart failure. Another aspect of the invention, then,
may be characterized as a method of adjusting the heart rate of a
patient by means of an implanted rate adaptive pacemaker, where
automatic adjustment of the pacing rate of the pacemaker is
achieved in response to a determination of the patient's intrinsic
impedance derived solely from the electrical energy generated by
the patient's heart. The patient's intrinsic impedance information
may be used instantaneously to influence the rate adaptation on a
continuous or ongoing basis within minutes. Alternatively, the rate
and cardio-pulmonary response derived from the intrinsic impedance
may be applied to determine the rate adaptation on a longer term
basis, such as on a daily or monthly basis.
It is therefore another important aim of the invention to provide a
system which uses an impedance derived parameter, such as
ventilation, as a closed loop parameter to optimize the rate
response of an implantable rate adaptive cardiac pacemaker or
defibrillator.
The invention may be further stated as allowing the
cardio-pulmonary status of a patient to be monitored with an
implantable monitoring device, by calculating the patient's
thoracic impedance based on information derived from the electrical
energy generated by the patient's own heart. For example, the
device may be implanted subcutaneously to monitor the patient's
EKG, and to detect changes in the thoracic impedance based on
differential signal processing of the EKG. Then, information
concerning the impedance changes may be applied within the device
to determine the cardio-pulmonary status of the patient. In
circumstances where the patient is suffering from heart failure,
the device is adapted to monitor the patient's heart failure by
performing the calculation of impedance and processing thereof
solely using the electrical energy generated by the patient's own
heart.
Information about the cardiac function of the patient may be
obtained using a method according to the invention, in which
electrical signal information from an EKG derived from
depolarization and repolarization of the patient's heart,
representing systole and diastole, accordingly for different phases
of the heart represented by the EKG, is continuously processed
using electrical energy generated by the patient's own heart. On
the other hand, the impedance of the patient's heart may be
analyzed with systole from a point close to the T-Wave of the EKG
signal, and information on the diastolic status of the heart may be
derived from an impedance signal at the R-Wave of the EKG signal.
Then, a comparison between systole and diastole may be used to
ascertain indirectly cardiac stroke volume and the cardio-pulmonary
status of the patient.
Similarly, the function of a body-implantable defibrillator may be
enhanced according to the invention by determining the impedance
between sensing electrodes of the defibrillator, and determining
changes in that impedance, based solely on energy generated by the
patient's heart.
It will be seen from the ensuing detailed description that a device
for evaluating the cardio-circulatory condition of a patient is
implemented with means for determining the patient's thoracic
impedance and impedance changes based on information solely derived
from the electrical energy generated by the patient's own heart.
And the desired information may be obtained by extremely simple
means so that the changes or additions required to achieve these
benefits with even currently available devices can be minimal, such
as including surface mounted electrodes for monitoring the
patient's surface EKG in subcutaneously implanted devices.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aims, objectives, aspects, features and
advantages of the invention will be better understood from a
consideration of the following detailed description of the best
mode contemplated for practicing the invention, taken with
reference to certain preferred implementations and methods, and the
accompanying drawings in which:
FIG. 1A-1D are schematic diagrams of an electrical circuit or
system in which a patient's heart is represented by an internal
resistance, useful for explaining the basic principle of the
present invention;
FIG. 2 applies the concepts discussed with reference to FIG. 1, to
the principle of different EKG vectors, FIG. 2A illustrating EKG
vectors measured for a patient, and FIG. 2B illustrating the EKG
tracings for different placements of EKG electrodes;
FIG. 3 illustrates various situations in which the internal
resistance of a patient's heart is represented in parts A, B, C and
D not as one single value, but by plural individual impedances;
FIG. 4 is a graph of EKG data and impedance factor versus time that
illustrates results from measurements taken with an intracardiac
electrode;
FIGS. 5 and 6 are graphs of EKG data and impedance factor vs. time
corresponding to FIG. 4, except for changes (respective increases)
in the respiratory rate;
FIG. 7 is a graph that summarizes and compares the measurements of
impedance derived ventilatory rate obtained using two different
methods;
FIG. 8 is a graph that depicts a preferred application of the
signal processing means, with with relative signal amplitudes over
time;
FIG. 9 is a graph that represents the quotient of raw voltage and
shunt voltage with different depth and modes of breathing;
FIG. 10 is a graph that depicts adjustment of pacing rate, in units
of pacing rate versus extent of a patient's physical activity
detected by a sensor; for rate responsive pacing;
FIG. 11 is a flow chart that shows the principle of a closed loop
rate profile optimization; and
FIG. 12 is a flow diagram of the principles and signal processing
of the present invention.
DETAILED DESCRIPTION OF THE PRESENTLY CONTEMPLATED BEST MODE OF
PRACTICING THE INVENTION
The method and device of the invention will be better understood by
reference to a presently preferred embodiment constituting a stand
alone diagnostic or therapeutic device to enhance the specificity
of a body-implantable device such as an artificial cardiac
pacemaker, a defibrillator, a cardiac resynchronization device, or
a monitoring device for evaluating the cardiopulmonary functional
status of the patient.
Throughout the views of the drawings, identical reference numbers
indicate identical structures. Views of the device or the method
either alone or implanted are not intended to represent the actual
or relative sizes or rather give principle understanding of the
methods and devices.
FIGS. 1A-1D are simplified schematic diagrams of an electrical
circuit or system in which the heart of a patient 1 is represented
by an internal resistance (R.sub.internal) 2, at least in parts 1A
and 1B. In principle, R.sub.internal is the sum of many individual
resistances or impedances consisting of myocardial tissue, fibrous
connective tissue in the heart, pericardium, blood within the
heart, fluids within the intracellular spaces, and the surrounding
environment of connective tissue atria, pulmonary structures,
venous structures, and lung tissue. The multiplicity of cells
within the heart depolarize, and this depolarization creates an
electrical force which threads through the heart with a certain
vector. This initial electrical source represented in the circuit
schematic of the Figures as a voltage source E (3) has a magnitude
that can be measured between electrodes at points A (4) and B (5).
The latter two points may constitute the sites of electrodes of an
external measuring instrument or EKG amplifier.
Virtually the same voltage as the original source voltage E 3 will
be present and remain so, if the impedance between measuring points
4 and 5 has a magnitude of several megohms. This is because under
that condition little or no current is flowing between those
points. In principle, this maximum voltage is present, for example,
at the input amplifier of an implantable pacemaker, since they have
an impedance of several hundred kilohms or megohms; and the same is
true of other implantable diagnostic devices, such as devices that
measure the patient's EKG. For example, external EKG strip chart
recorders or EKG monitors have an impedance of 1 megohm or more,
thereby allowing them to detect the maximum voltage present between
points of the body at which their electrodes are attached or
located.
In FIG. 1B the schematic shows a heart 1, in circuit with an
internal resistance 2, a source voltage 3 that is detected between
electrode points or sites 4 and 5, corresponding to what has been
described above for FIG. 1A. However, in this case an additional
external resistance of magnitude R.sub.Load 6 is connected in
circuit between electrode points 4 and 5. If the magnitude of the
resistance of load 6 is considerably lower than the input impedance
between 4 and 5 represented by, say, the virtual open circuit
impedance of an amplifier, discussed for the circuit of FIG. 1A,
considerable current will be shunted through this lower resistance.
As in the case of a failing battery, then, the initial full voltage
E (3) will not be detected since, by Ohm's Law (U=IR), a drop in
voltage will be observed with decreased R.
This example is carried forward in FIGS. 1C and 1D. In each of
those Figures, the heart 1 generates a certain EKG signal 7 between
measuring points 4 and 5, which has a certain amplitude 8. In FIG.
1D, the connection of an input amplifier 8 with a relatively low
resistance R.sub.Load 6 across points A and B results in an EKG
signal 7 having an amplitude considerably lower than that in the
case of the much higher, virtually open circuit resistance across A
and B in FIG. 1C.
In principle this observation can be compared to a battery. When a
battery fails it is typically because its internal resistance has
increased to a level that no longer supports the supply of a useful
amount of electrical energy to an external load. If one measures
the voltage V of a failing battery which is disconnected, it is
usually found that the battery has a nearly normal voltage because
a conventional voltmeter used to perform the measurement has an
input resistance much higher than the internal resistance of the
battery. If, however, the failing battery is connected to a low
external resistance such as load 6 in FIG. 1B, the terminal voltage
U of the battery drops precipitously. This can be interpreted as
the battery dropping most if its source voltage across its own
internal resistance, so little or no voltage is available for
external services. For example, an ideal battery with 0 internal
resistance or infinite internal conductance and a voltage E of 12
volts, when supplying power to an external load having a resistance
of 1 ohm, will produce a current of I=12 amps and a power of
E.times.I=144 watts. If the battery has an internal resistance of 2
ohm, or an internal conductance as low as 0.5 siemens, then with
this load, the terminal voltage U of the battery will drop to 4
volts. The output current of this failing battery will drop to
I=U/R.sub.Load which is 4 amps, and the output power is 16
watts.
The same principle holds for conventional electrocardiography along
the main electrical excitation vector. Since an EKG measurement is
detected with high input impedance, this conventional measurement
gives no insight into the electrical power of the source, in this
case the heart of the patient. Furthermore, the absolute voltage of
EKG signals is not a valuable indicator of various pathological
situations. Indeed, despite great diversity of cardiac diseases it
is common clinical experience that individual variability and the
amplitude of the EKG wave as detected from state of the art EKG
amplifiers is not indicative of any kind of disease. The voltage of
a conventional EKG is reduced only in very few clinical situations
when large electrical shunts are present, such as a pericardial
effusion which constitutes a large conductor around the heart that
shunts the electrical energy with low intrinsic resistance.
The terminal voltage U in FIG. 1B (which represents the R wave
amplitude 8, represented in FIG. 1C by QRS-complex 7) should drop
by E-(R.sub.internal.times.I)), where I=U/R.sub.Load. The internal
resistance R.sub.internal 2 can be calculated by the equation
R.sub.internal=(V-U)/(U/R.sub.Load), where V is the voltage between
electrode points 4 and 5 which is disconnected from the load in
FIG. 1A and U is the terminal voltage between 4 and 5, across which
the electrical load R.sub.Load 6 is connected. Based on
R.sub.internal, the internal electrical conductance S.sub.internal
of the heart can also be calculated by the equation:
S.sub.internal=1/R.sub.internal, in siemens.
These equations are applied in practice in FIG. 2, according to the
principle of different EKG vectors. FIG. 2A represents a patient 9
in which EKG lead I 10 is represented by electrodes to the
patient's right and left arms, with voltage measurements shown in
EKG tracing 11 (FIG. 2B). Electrode detection lead/electrodes II 12
produces the tracing 13, and the voltage detected between the
electrodes of lead III 14 is shown in the tracing 15 for lead III.
Tracings 11, 13, and 15, then, represent the EKG signal obtained
for the respective vector projections of the leads 10, 12 and 14.
If an external load is now connected in parallel with the input
impedance of the EKG amplifier, a voltage drop will be observed for
the same patient as shown by tracings 16 (for lead I), 17 (for lead
II), and 18 (for lead III), because part of the energy delivered
from the patient's heart is shunted through the additional external
load.
FIG. 3 illustrates various situations in which the internal
resistance R.sub.internal of the heart is represented not as one
single value, but by plural individual impedances. In FIG. 3A, the
internal resistance R.sub.heart of the heart 19 is represented by
the structural resistance of the heart made up of cells, connective
tissue, and primarily solids, and a variable component of
resistance R.sub.inspiration (or R.sub.insp) 20 is primarily
represented by the filling of the heart with blood. Since blood has
a specific impedance of roughly 50 ohms per centimeter (cm), while
the specific impedance of heart 19 is 400 ohms per cm, there exists
a great influence on the total impedance of the heart, because
these two components are in parallel. The internal voltage source 3
in FIG. 3A detected between electrode points 4 and 5 represents a
voltage V.sub.1 (22) that equals primarily E 3 if the input
impedance between 4 and 5 is sufficiently high that all of the
voltage E drop occurs between 4 and 5.
If, however, an additional external load R.sub.load 6 of roughly 1
kilohm or less is applied as shown in FIG. 3B, then the voltage
between 4 and 5 drops to a voltage U1 (23) which, as earlier
described, is lower than the voltage E 22. FIG. 3C illustrates the
situation in which a variation now occurs in internal resistance
21, representing the resistance with expiration (R.sub.expiration
or R.sub.exp). The total voltage to be detected V.sub.2 24 is now
primarily composed of the parallel resistances of R.sub.heart 19
and R.sub.exp 21. A variation between R.sub.insp 20 and R.sub.exp
21 will not affect the resulting voltages, V.sub.2 24 or V.sub.1
22, since the input impedance between electrodes 4 and 5 is
sufficiently high to avoid further voltage shunting and voltage
drop. However, as shown in FIG. 3D, the external load 6 will affect
voltage U.sub.2 25 with a variation in internal impedance 21 during
expiration, compared to impedance component 20 during inspiration.
Thus, if an external load 6 of sufficient load resistance, such as
1 kilohm, is applied to a primarily high input impedance amplifier,
variations in internal total resistance build up from R.sub.heart
19 and R.sub.respiration 20 or 21 have a much greater effect on
voltage U.sub.1 23 with inspiration and U.sub.2 25 with expiration.
In principle, for this condition it can be said that
R.sub.inspiration is not identical with R.sub.expiration and
therefore, U.sub.1 23 is different from U.sub.2 25. It follows that
U.sub.1 equals the delta of U.sub.2, and this represents more or
less the impedance factor of respiration, the term "impedance
factor" meaning the quotient of impedances 19 and 20 in FIG. 3B
compared to the impedances 19 and 21 in FIG. 3D.
FIG. 4 is a graph of EKG data and impedance factor versus time that
illustrates results from measurements taken with an intracardiac
electrode. A bipolar conventional pacemaker electrode was implanted
in the heart and measurements were taken between the electrode tip
in connection with the myocardium and a ring located roughly 1 cm
behind the electrode tip. These sites can be considered as
electrode points 4 and 5 in the Figures described thus far, and a
linear high quality amplifier was connected between these two
sites. The signal processing was performed in such a way that one
signal represented in FIG. 4 as raw voltage 26 represented by the
higher bars in the graph was compared to a shunt voltage 27
represented by the smaller bars. To detect the shunt voltage from
the same electrode site 4 and 5 by a special program, the input
impedance was shunted by a resistance of one kilohm. In FIG. 4 the
time axis (abscissa) 28 shows increments of time in seconds and the
voltage axis (left ordinate) 29 shows increments of the detected
voltage of the two signals raw voltage 26 and shunt voltage 27. The
curve 30 represents the quotient between voltage 26 and voltage 27
(i.e., their impedance factor, measured along lines parallel to the
right ordinate) or in other words, the quotient of the impedances
that change with respiration.
As is clearly seen in the graph, the ratio of the peak signal
between raw voltage 26 and shunt voltage 27 represented by curve 30
correlates with the respiration, which was set to 5.5 cycles per
minute. The time interval for one respiratory cycle is 11 seconds
in this example, which actually represents a ventilation rate of
5.5 cycles per minute.
Various aspects of the continuous EKG signal can be used to derive
measurements of impedance factor in FIG. 4, to discern or determine
the cardio-pulmonary status of the patient using, in this example,
the bipolar conventional pacemaker electrode implanted in the heart
for monitoring purposes. Either a continuous line can be averaged
if a sufficiently high digitization rate is applied, or, to
simplify measurements and procedures, and also to facilitate data
handling and power consumption in an implantable device, only
certain aspects of the EKG signal need be taken. For example, the
latter aspects may be those represented previously herein in EKG
signal 7 with amplitude 8, so it is feasible to use only the peak
7A of the R Wave or to take other aspects such as only or
additionally the T-Wave peak 7B, of the EKG signal 7 illustrated in
FIG. 2B. In the example shown in FIG. 4, the peak of the R-Wave was
applied. From the latter Figure, it is clear that considerable
variation occurs in the quotient represented by curve 30 (the
impedance factor) between inspiration and expiration, which
corresponds to the ventilatory cycle rate and its amplitude.
FIGS. 5 and 6 are graphs illustrating the same data setting and the
same parameters as in FIG. 4; however, the respiratory rate was
changed in FIG. 5 to 10 cycles, and in FIG. 6 to 20 cycles per
minute. This change in frequency is clearly shown in the latter two
Figures, being represented by peak ratio 30.
FIG. 7 is a graph that summarizes and compares the measurements of
true ventilatory rate V along the abscissa and of the impedance
factor derived ventilatory rate Z along the ordinate. As shown,
there is nearly a 1:1 correlation between the two methods which
means that ventilation can be truly detected, as far as the
frequency of breaths is concerned, from the impedance derived
signal in which the heart serves as the power source for the
impedance calculation.
FIG. 8 depicts a preferred application of the signal processing
means, with the abscissa showing units of time and the ordinate
showing the signal amplitudes of the raw voltage 26 and the shunt
voltage 27 relative to one another. The former is indicated by the
higher amplitude and the latter, derived from the shunted input
impedance which was reduced to 0.5 kilohm, is indicated by a lower
amplitude. The signal processing of both signals 26 and 27 included
passage through a low pass filter for smoothing.
The application of low pass and high pass filtering in the signal
processing can be performed by either a conventional analog
technique or also by conventional digital technique of first,
second, third or fourth order. The selection of the cut off
frequency depends on preference as to which signal components
should be detected. If it is preferred to detect respiration and
respiration rate by filtering with a cut off frequency below twice
the maximum frequency that is expected. For most patients, a
respiratory rate of no more than 50 breaths per minute can be
expected, which means that a low pass filter of about 1.5 Hz or
less will allow detecting the respiratory rate. In FIG. 8 the
difference in signal amplitude between signal 26, which was
detected from input impedance of more than 1 megohm, and signal 27
detected from an input impedance of 0.5 kilohm, becomes evident
from FIG. 8. The respiratory cycle rate is also evident.
FIG. 9 is a graph that represents the quotient of raw voltage and
shunt voltage with different depths or amplitudes and modes of
breathing. This graph shows that it is not only feasible to detect
respiratory rate, which is effected by change in the filling of the
heart with blood and by a volume change of the amount of blood
surrounding an electrode implanted in the heart with a conductor
such as blood, but it is also feasible to detect a relative change
in amplitude following different tidal volumes. Graph 31 represents
the signal derived from the impedance quotient of high input
impedance of more than 1 megohm and 1 kilohm with external
artificial ventilation of an individual with a tidal volume of 300
ml per breath. Line 32 shows the same for a tidal volume of 850 ml.
Line 33 depicts the impedance quotient with spontaneous breathing
at a considerably lower rate. Now the signal shows not only the
breathing or ventilation, but also the cardiac component 33A
indirectly reflecting stroke volume with systole and diastole. In
order to obtain this, for example, one need only observe the
depolarization that occurs with the electrical signal represented
by the peak of the R wave 7A in the EKG or relative to the
repolarization that occurs with the peak of the T wave 7B,
recognizing that the mechanical contraction occurs slightly after
the peak of the R wave. At this point, intracardiac impedance
allows determining the extent of filling of the heart and to derive
therefrom indications of heart failure.
FIG. 10 is a graph that depicts, in units of pacing rate versus
extent of physical activity detected by a sensor, the principle
currently used for rate responsive (or rate adaptive) pacing in
cardiac pacemakers and in defibrillators, to adjust the heart rate
or the pacing rate of a patient based on the input received from a
sensor (such as an accelerometer in heretofore available devices).
In the Figure, sensor units 35, which may, for example, be activity
counters, amplitude of an activity signal, or any other
physiological parameter which has a correlation to heart rate in
healthy persons such as body temperature, ventilation, or
mechanical forces acting on the body. These units are translated in
a slope function into a rate response with more sensor units
yielding a high pacemaker rate 34. Based on empirical assumptions a
certain slope is selected for a given patient. Slope A (36)
represents a correlation of sensor units and pacemaker rate for a
physically fit patient, while a slope B (37) is selected for a less
physical patient. On average there is a threshold on which the
heart rate is increased until maximum exertion 38 and maximum heart
rate 34 are reached.
The limitation with all these open loop systems of the prior art is
that the slope that sets an individual rate with a given exercise
is selected on an empirical basis; however, it is not confirmed
that this heart rate is the optimum heart rate for the given
situation of a patient. Even in the same patient, the most
beneficial rate with a given exercise might change from day to day.
Some patients have significant coronary heart disease which limits
the flow of blood through the coronary arteries and therefore
induces an ischemia Therefore, it is sometimes beneficial to limit
the maximum heart rate and the slope to different values (e.g., 39)
compared to a state that might have been present weeks or even only
days ago when the myocardial perfusion was different. The lack of a
feedback parameter is one of the limitations of currently
widespread use of rate adaptive pacemakers.
Considering the limitation of currently open loop rate regulation
implantable pace makers, an impedance derived parameter according
to the invention can be used not only to adjust the pacing rate
directly so as to create a correlation between signal and pacing
rate on a linear or nonlinear basis as has been suggested in the
past by ventilation control rate adaptive pacemakers, but it also
can be used to control the individually optimum pacing and heart
rate on a closed loop basis, and also on a long term trend basis.
In the past it has been considered that hemodynamic parameters
would be suitable for a closed loop system. However, the limitation
is to derive hemodynamic parameters directly. The complexity and
change in those parameters and the technical difficulty to measure
an actual derivative of cardiac output or stroke volume has
prevented an introduction of those practices into clinical
practice. In stark contrast, the present invention provides a novel
minimally energy demanding system that allows controlling the
effect of pacing rate and the adequacy of pacing rate by monitoring
the ventilation and/or cardiac parameters derived from an impedance
signal which is obtained in a manner according to the
invention.
FIG. 11 is a flow chart that shows the principle of a closed loop
rate profile optimization. In this optimization, the impedance
derived rate controlled parameter is processed and stored in
circuitry 40 which would process the signals obtained and store
them in a conventional memory means of an implantable device such
as an implantable pacemaker or defibrillator. This can be done
either as a long term average over several days, or over a shorter
term average over a daily or even an hourly or minute-by-minute
basis. The difference between long term and short term averages as
described in previous patents by the applicants referenced earlier
herein may be applied to this process. The actual rate profile is
available from the second circuitry 41. This actual rate profile is
compared to the long term impedance derived rate control parameter
in a comparator and logic circuit 42, and the output is used to
adjust the heart rate setting at 43. This not only controls the
slope, for the correlation of pacing rate to a given signal
intensity or work load as shown in the graph 44, but may also be
used to set a new target rate and base rate. In this way, an
optimization of the rate profile and slope can be achieved by means
of minimal additional hardware complexities, since the EKG
electrodes, the EKG amplifier and the information is already
available to any demand pacemaker and defibrillator.
Alternatively, the signal can be also processed on a short-term, so
that the effect of the heart rate adjustment can be evaluated
against the ventilation and cardiac response within tens of seconds
to achieve an individual optimization by circuitry 42. The pacing
rate that gives the lowest ventilation response is the optimum for
a given work load. It is known that a person who cannot increase
his heart rate will breathe more heavily than one who has the more
adequate heart rate. Experiments done by the applicants have shown
in the past that if the pacing rate is kept constant at 70 bpm and
there is no further rate increase with exercise, the exercise
capacity of a patient is limited and he/she will breathe heavily
despite the limited exercise capacity. Therefore, the response of
ventilation can be taken as an indirect indicator of the metabolic
load and of the efficiency of the cardio-circulatory system. In
this way, the invention fulfills an aim to provide a system that
uses ventilation as a closed loop parameter to optimize the rate
response given brought forward by a different sense of parameter.
In addition, ventilation can also be used as a primary parameter to
adjust the heart rate with or without secondary optimization
provided through another sense parameter.
FIG. 12 is a flow diagram of the principles and signal processing
of the present invention. An EKG signal sensor 45 consists of
electronic means that makes connection with the site where the EKG
signal is sensed either merely on an intracardiac basis, from a
bipolar electrode, from a unipolar electrode (one electrode in the
heart and one on the case), or from separate electrodes on an
implantable case such as surface-mounted electrodes on a pacemaker,
a defibrillator, or monitoring device that is preferably implanted
subcutaneously, or implanted in a different location in the body as
described in the cross-referenced related applications. At least
bipolar EKG signals are sensed, and by a switching relay process 46
a load 6 is either added or not added to the signal circuitry.
Thereby, two different EKG derived information signals are
available for EKG amplifier and processor 47, one signal with and
one signal without the load. Comparator and analyzer 48 derives the
desired information from the quotient of the two impedances that
correspond to voltages and impedances derived from a high and a low
input impedance. This information is then provided on either a
filtered or processed basis to storage medium 49 in which several
long term, short term, and derivatives of the signals can be
stored. Comparator 50 finally derives the information that is
required to the respective implantable device either for a rate
profile adjustment and optimization 51, or if it is only for
monitoring purposes, for congestive heart failure 52 as described
as one of exemplary embodiments in the aforementioned related '184
patent application. This telemonitoring signal can be telemetered
to an outside device or through telemetry 53 or can also provide
patient alert 54 in case of detecting a deviation from the desired
pattern for the individual patient. Electronic means applied are
state of the art, with switching relay process, EKG signal sensor,
amplifier processor, analyzer, storage and comparator provided
either in a single chip or the like, or in conventional electronic
components applying a combination of digital and analog techniques
or in solely digital techniques including filtering and effecter 55
that converts the available information into the desired action
within the device.
In summary, the current invention provides a facilitated monitoring
means to optimize both therapeutic and diagnostic capacity of an
implantable device by means of impedance derived information, and
to acquire this information using the patient's own heart to
provide the necessary electrical energy.
Although a presently contemplated best mode of practicing the
invention has been disclosed by reference to certain preferred
methods, it will be apparent to those skilled in the art from a
consideration of the foregoing description that variations and
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, it is intended that the
invention shall be limited only by the appended claims and the
rules and principles of applicable law.
* * * * *